In multi-layered circuit card assemblies (also known as printed wiring assemblies or circuit board assemblies), component leads are connected to the layers by forming holes through the card assembly at selected positions; locating the leads in the holes; and soldering each of the leads to all of the layer portions surrounding the holes. Circuits extend to selected layer portions to form the complete circuit card assembly.
It is important to determine whether the solder joints are defective or good. It is known to inspect solder joints with a laser inspection system including a laser and a detector. The laser is operated to apply a laser beam to the solder joint to heat the joint for a given time and the laser beam is switched off with a shutter mechanism. The hot solder joint emits infra red emission which is measured to determine the cooling properties of the joint. The cooling rate determines the condition of the solder joint as to whether the solder joint is defective or good. This process is disclosed in U.S. Pat. Nos. 4,696,104; 4,657,169; and 3,803,413. Reference also is made to 1986-ASOC Quality congress transaction--Anaheim, "Solder Joint Inspection Using a `Laser` Inspector", J. P. Streeter, pp 507-515; Printed Circuit World Convention III, May, 1984, "Automatic Laser Inspection System for Solder Joint Integrity Evaluation", Dr. Riccardo Vanzetti; and Electrionics, October 1985, "Overview of Laser/Thermal system Used to Detect Faulty Solder Joints", Alan C. Traub. Heretofore, the method of deciding correct shutter speed (In this application shutter speed is defined as the time duration which the laser beam is fired on the solder joint) has been based on microsectioning the solder joints and determining the correct shutter speed on classification of defects found during microsectioning. For example, large voids in solder joints are considered as a higher level of defect whereas small voids are considered as a lower level of defect. The major disadvantage of this method of inspection is that the laser shutter speed cannot be set without data obtained from microsectioning first (a destructive method).
At lower laser power, a solder joint is heated uniformly together with the lead yielding a false determination of the condition (defective or good) of a solder joint.